Mobile Me provides a separate email address (
[email protected]). You can have your Me account collect mail from other accounts (like your AT&T account) so everything appears in your Me inbox. Mobile Me is an IMAP email service, which is nice because your mail updates no matter where you access it from (whether webmail, your iMac or your Macbook), that is, once you read an email on the Macbook (for example), it will appear as "read" when you access mail later from your iMac or the web.
Syncing is probably the best feature of Mobile Me, IMO, because your contacts, calendar, Safari, etc., will be the same on your iMac and Macbook all the time. You will initially tell Mobile Me and each computer which computer's data is the one to take priority (ie., overwrite the others). Typically, you would make one computer's data (say, the iMac) up to date and the baseline (calendar, contacts, etc), tell it to override Mobile Me, then tell the other computer (the Macbook in this example) to be overwritten by what's on Mobile Me during the first sync -- this way everything looks like the original data on the iMac. After that, Mobile Me will sync both computers' data on the schedule you dictate in the options (hourly, daily, weekly, manually, etc).
There are other, cheaper ways to get a lot of the functionality of Mobile Me, but nothing that allows the truly effortless sync'g of all data between multiple Macs (and iPhones and iPod Touches if you have them).
I tried SyncTogether from Mark Space for over a year, and it works OK but is not great, and requires a bit of tweaking "nodes" etc. , and doesn't do anything for iPhones. Mobile Me works well (after the initial buggy launch).
FWIW, I don't use the Mobile Me mail account. I use Gmail's applications to get their IMAP mail and use my own domain -- this is all free and let's me have IMAP mail with my personal domain name (that is,
[email protected]).
Mobile Me is not perfect, and has shortcomings (a biggie is you cannot sync subscribed calendars -- that is, I can sync my calendar, by not my wife's to which I subscribe in iCal), but if you want your contacts, calendar, mail, bookmarks, etc., in sync between Macs, iPhones, and Touches without having to think about it, it's a good (albeit pricey) service.